Magnetoresistive element and method of manufacturing the same

ABSTRACT

According to one embodiment, a magnetoresistive element manufacturing method is provided. In this magnetoresistive element manufacturing method, a first ferromagnetic layer, tunnel barrier layer, and second ferromagnetic layer are sequentially formed on a substrate. A conductive hard mask is formed on the second ferromagnetic layer. The hard mask is patterned. A hard layer is formed on the side surface of the hard mask. The second ferromagnetic layer, tunnel barrier layer, and first ferromagnetic layer are processed by IBE in an oblique direction by using the hard mask and hard layer as masks. The IBE etching rate of the hard layer is lower than that of the hard mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-000671, filed Jan. 5, 2012, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetoresistiveelement and a method of manufacturing the same.

BACKGROUND

A spin transfer torque magnetic random access memory (MRAM) including amagnetoresistive element containing a ferromagnetic material as a memoryelement has been proposed. This MRAM is a memory that stores informationby controlling the electrical resistance of the magnetoresistive elementto two states, i.e., a high-resistance state and low-resistance state bychanging the magnetization direction in a magnetic layer by an electriccurrent to be injected into the magnetoresistive element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a memory cell of an MRAM;

FIG. 2 is a sectional view showing the structure of the memory cell ofthe MRAM;

FIG. 3 is a sectional view showing the structure of a magnetoresistiveelement according to the first embodiment;

FIGS. 4, 5, 6, and 7 are sectional views showing the manufacturing stepsof the magnetoresistive element according to the first embodiment; and

FIGS. 8, 9, 10, and 11 are sectional views showing the manufacturingsteps of a magnetoresistive element according to the second embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a magnetoresistive elementmanufacturing method is provided. In this magnetoresistive elementmanufacturing method, a first ferromagnetic layer, tunnel barrier layer,and second ferromagnetic layer are sequentially formed on a substrate. Aconductive hard mask is formed on the second ferromagnetic layer. Thehard mask is patterned. A hard layer is formed on the side surface ofthe hard mask. The second ferromagnetic layer, tunnel barrier layer, andfirst ferromagnetic layer are processed by IBE in an oblique directionby using the hard mask and hard layer as masks. The IBE etching rate ofthe hard layer is lower than that of the hard mask.

A magnetoresistive element such as a magnetic tunnel junction (MTJ)element using the tunneling magnetoresistive (TMR) effect has a pillarstructure including a reference layer and storage layer stacked with atunnel barrier layer being sandwiched between them. Each layer formingthis magnetoresistive element contains a hard metal material. This makesit difficult to process the magnetoresistive element into the pillarstructure by reactive ion etching (RIE) or the like. Consequently,plasma damage occurs, and the characteristics deteriorate. Whenprocessing the magnetoresistive element, therefore, physical etchingsuch as ion beam etching (IBE) using argon (Ar) is performed. In thisIBE processing, a hard mask used as a mask is removed (etched) by anamount corresponding to the etching amount of the magnetoresistiveelement and the etching selectivity (etching rate) to themagnetoresistive element.

In the IBE processing, the etched metal material adheres as a so-calledre-deposition product on the circumferential (side) surface of thepillar structure. This forms a leakage path over the reference layer andstorage layer. To prevent the formation of this re-deposition product,it is necessary to make an ion beam of IBE incident in a direction(oblique direction) inclined to the perpendicular direction (normal) ofthe substrate surface, and perform etching while removing there-deposition product on the circumferential surface of the pillarstructure.

In this case, the above-described removal of the hard mask progressesnot only in the vertical direction but also in the oblique direction.That is, not only the upper surface of the hard mask is etched in thevertical direction, but also the shoulders (corners) of the hard maskare etched in the oblique direction. The influence of this obliqueetching becomes significant as the micropatterning of the MTJ advances.Especially when the diameter of the pillar structure of amagnetoresistive element to be processed becomes smaller than its height(film thickness), most of the removal amount of the hard mask is theamount of oblique etching. Consequently, the hard mask is shaped into acone whose diameter decreases from the lower portion toward the upperportion, and does not function as a mask any more. Accordingly, nopillar structure having a desired shape can be obtained.

To solve the above problem, it is possible to make the film thickness ofthe hard mask much larger than that of the magnetoresistive element. Ifthe film thickness of the hard mask is increased, however, theprocessing (patterning) of the hard mask itself becomes difficult, andthis produces dimensional variations of the hard mask andmagnetoresistive element. The dimensional variations exert a very largeinfluence on the characteristics of the magnetoresistive element. Also,if the film thickness of the hard mask is increased, the hard maskbecomes an obstacle to an obliquely entering ion beam. Since this makesit necessary to increase the distance between adjacent magnetoresistiveelements, it is difficult to increase the density of elements.

Another method is to use, as the hard mask material, a hard material (amaterial having a high Vickers hardness) having an IBE etching rate muchlower than that of the magnetoresistive element. Examples of the hardmaterial are insulating materials such as diamond-like carbon (DLC),microcrystalline alumina (Al₂O₃), boron nitride (BN), and siliconcarbide (SiC). However, the hard mask must be a metal material in orderto electrically connect the magnetoresistive element and an upperelectrode after the magnetoresistive element is processed. Accordingly,none of the above-mentioned hard materials can be used as the hard mask.

A metal such as titanium nitride (TiN), tantalum (Ta), or tungsten (W)cannot sufficiently increase the etching rate to the magnetoresistiveelement. As described above, therefore, the film thickness must beincreased even when using any of these metal materials as the hard mask.This produces dimensional variations and characteristic deterioration ofthe magnetoresistive element, and makes the density of elementsdifficult to increase.

By contrast, this embodiment solves the above problems by using a metalmaterial as the hard mask in the IBE processing of the magnetoresistiveelement, and making the etching rate of the hard mask lower than that ofthe magnetoresistive element.

This embodiment will be explained below with reference to theaccompanying drawing. In the drawing, the same reference numerals denotethe same parts. Also, a repetitive explanation will be made as needed.

<MRAM Configuration Example>

A configuration example of an MRAM will be explained with reference toFIGS. 1 and 2.

FIG. 1 is a circuit diagram showing a memory cell of the MRAM.

As shown in FIG. 1, a memory cell in a memory cell array MA includes aseries-connected circuit of a magnetoresistive element MTJ and aswitching element (for example, an FET) T. One terminal of theseries-connected circuit (one terminal of the magnetoresistive elementMTJ) is connected to a bit line BLA, and the other terminal of theseries-connected circuit (one terminal of the switching element T) isconnected to a bit line BLB. The control terminal of the switchingelement T, for example, the gate electrode of the FET is connected to aword line WL.

A first control circuit 11 controls the potential of the word line WL. Asecond control circuit 12 controls the potential of the bit lines BLAand BLB.

FIG. 2 is a sectional view showing the structure of the memory cell ofthe MRAM.

As shown in FIG. 2, the memory cell includes the switching element T andmagnetoresistive element MTJ arranged on a semiconductor substrate 21.

The semiconductor substrate 21 is, for example, a silicon substrate, andits conductivity type can be either a p- or n-type. A silicon oxide(SiO₂) layer having an STI structure is formed as an element isolationinsulating layer 22 in the semiconductor substrate 21.

The switching element T is formed in the surface region of thesemiconductor substrate 21, i.e., in an element region (active area)surrounded by the element isolation insulating layer 22. In thisexample, the switching element T is an FET, and includes twosource/drain diffusion regions 23 in the semiconductor substrate 21, anda gate electrode 24 formed on a channel region between them. The gateelectrode 24 functions as the word line WL.

The switching element T is covered with an interlayer dielectric layer(for example, SiO₂) 25. A contact hole is formed in the interlayerdielectric layer 25, and a contact via (CB) 26 is formed in this contacthole. The contact via 26 is made of a metal material such as W or copper(Cu).

The lower surface of the contact via 26 is connected to the switchingelement. In this example, the contact via 26 is in direct contact withthe source/drain diffusion layer 23.

A lower electrode (LE) 27 is formed on the contact via 26. The lowerelectrode 27 has, for example, a multilayered structure including Ta (10nm)/ruthenium (Ru) (5 nm)/Ta (5 nm).

The magnetoresistive element MTJ is formed on the lower electrode 27,i.e., immediately above the contact via 26. Details of themagnetoresistive element MTJ according to this embodiment will bedescribed later.

An upper electrode (UE) 28 is formed on the magnetoresistive elementMTJ. The upper electrode 28 is made of, for example, TiN. The upperelectrode 28 is connected to the bit line (for example, Cu) BLA througha via (for example, Cu) 29.

First Embodiment

A magnetoresistive element MTJ according to the first embodiment will beexplained below with reference to FIGS. 3, 4, 5, 6, and 7. The firstembodiment is an example in which in the processing steps of themagnetoresistive element MTJ, a hard layer 34 is formed on thecircumferential (side) surface of a hard mask 33 made of a metalmaterial, and the magnetoresistive element MTJ is processed by physicaletching using these hard mask and hard layer as masks. This makes itpossible to reduce the dimensional variations of the hard mask 33 andmagnetoresistive element MTJ. The magnetoresistive element MTJ accordingto the first embodiment will be explained in detail below.

Structure of First Embodiment

First, the structure of the magnetoresistive element MTJ according tothe first embodiment will be explained below with reference to FIG. 3.

FIG. 3 is a sectional view showing the structure of the magnetoresistiveelement MTJ according to the first embodiment. FIG. 3 shows thestructure of two adjacent magnetoresistive elements MTJ.

As shown in FIG. 3, the magnetoresistive element MTJ includes a storagelayer 30, tunnel barrier layer 31, and reference layer 32.

The storage layer 30 is formed on an underlayer (not shown) on a lowerelectrode 27. The storage layer 30 is a ferromagnetic layer in which themagnetization direction is variable, and has perpendicular magnetizationthat is perpendicular to or almost perpendicular to the film surfaces(upper surface/lower surface). “The magnetization direction is variable”herein mentioned means that the magnetization direction changes withrespect to a predetermined write current. Also, “almost perpendicular”means that the direction of residual magnetization falls within therange of 45°<θ≦90°.

The storage layer 30 is made of a ferromagnetic material containing oneor more elements selected from, for example, cobalt (Co) and iron (Fe).It is also possible to add an element such as boron (B), carbon (C), orsilicon (Si) to the ferromagnetic material, in order to adjustsaturation magnetization, magnetocrystalline anisotropy, or the like.

The tunnel barrier layer 31 is formed on the storage layer 30. Thetunnel barrier layer 31 is a nonmagnetic layer and made of, for example,magnesium oxide (MgO).

The reference layer 32 is formed on the tunnel barrier layer 31. Thereference layer 32 is a ferromagnetic layer in which the magnetizationdirection is invariable, and has perpendicular magnetization that isperpendicular to or almost perpendicular to the film surfaces. “Themagnetization direction is invariable” herein mentioned means that themagnetization direction remains unchanged with respect to apredetermined write current. That is, a magnetization direction reversalthreshold value of the reference layer 32 is larger than that of thestorage layer 30.

The reference layer 32 is made of a ferromagnetic material containingone or more elements selected from, for example, Co, Fe, B, nickel (Ni),iridium (Ir), platinum (Pt), manganese (Mn), and Ru.

The film thickness (height) of the magnetoresistive element MTJincluding the storage layer 30, tunnel barrier layer 31, and referencelayer 32 is, for example, 40 (inclusive) to 50 (inclusive) nm. Theplanar shape of the magnetoresistive element MTJ is, for example, acircle, and the diameter of the circle is, for example, 30 (inclusive)to 50 (inclusive) nm. Therefore, the magnetoresistive element MTJ isformed into a pillar shape.

Note that although not shown, an interface layer may also be formed inthe interface between the reference layer 32 and tunnel barrier layer31. The interface layer is formed to obtain lattice matching with thetunnel barrier layer 31 in contact with the lower surface of theinterface layer. The interface layer is made of, for example, the samematerial as that of the reference layer 32, but the composition ratiocan be different from that of the reference layer 32.

A shift adjusting layer can be formed on a spacer layer (for example,Ru) (not shown) on the reference layer 32. The shift adjusting layer isa magnetic layer in which the magnetization direction is invariable, andhas perpendicular magnetization that is perpendicular to or almostperpendicular to the film surfaces. The magnetization direction isopposite to that of the reference layer 32. This enables the shiftadjusting layer to cancel a leakage magnetic field from the referencelayer 32 to the storage layer 30. In other words, the shift adjustinglayer has an effect of adjusting the offset of the reversalcharacteristic applied on the storage layer 30 by a leakage magneticfield from the reference layer 32 in the opposite direction. This shiftadjusting layer is made of, for example, an artificial lattice having amultilayered structure containing a magnetic material such as Ni, Fe, orCo and a nonmagnetic material such as Cu, Pd, or Pt.

The storage layer 30 and reference layer 32 can have a dimensionaldifference in a plane. For example, the planar diameter of the storagelayer 30 can be smaller than that of the reference layer 32. This makesit possible to prevent an electrical shortcircuit between the storagelayer 30 and reference layer 32. Note that the planar shape of themagnetoresistive element MTJ is not limited to a circle, and may also bea square, rectangle, or ellipse.

Furthermore, the positions of the storage layer 30 and reference layer32 can be switched. That is, the reference layer 32, tunnel barrierlayer 31, and storage layer 30 can be formed in this order on the lowerelectrode 27.

In this embodiment, the hard mask 33 and hard layer 34 are formed on themagnetoresistive element MTJ.

More specifically, the hard mask 33 is formed on the reference layer 32.The hard mask 33 is made of a conductive metal material, for example,TiN. The hard mask 33 may also be a film containing Ti, Ta, or W, or amultilayered film containing these materials.

A film thickness (height) H of the hard mask 33 is, for example, 40(inclusive) to 50 (inclusive) nm. The planar shape of the hard mask 33is, for example, a circle, and the diameter of the circle is, forexample, 20 (inclusive) to 30 (inclusive) nm. Accordingly, the hard mask33 is formed into a pillar shape.

The hard layer 34 is formed on the circumferential surface of the hardmask 33. In other words, the hard layer 34 is formed to cover thecircumference of the pillar hard mask 33. The hard layer 34 is made of,for example, a hard material having an IBE etching rate lower than thatof the hard mask 33 (for example, TiN, Ta, or W), in a manufacturingstep to be described later. Examples of this hard material areinsulating materials such as DLC, microcrystalline Al₂O₃, BN, and SiC.Note that the IBE etching rate of the hard layer 34 is desirablyone-third or less that of the hard mask 33.

The IBE etching rate is mainly determined by the Vickers hardness of thematerial. The Vickers hardness of any of the above-mentioned hardmaterials is 1,000 or more. By contrast, the Vickers hardness of thehard mask 33 is, for example, about a few hundreds. That is, the Vickershardness of the hard layer 34 is higher than that of the hard mask 33.This can make the IBE etching rate of the hard layer 34 higher than thatof the hard mask 33. More specifically, the Vickers hardnesses of DLC,Al₂O₃, BN, and SiC are respectively about seven, three, six, and threetimes that of the hard mask 33 (TiN). Accordingly, the IBE etching rateof any of the above-mentioned hard materials can be one-third or lessthat of the hard mask 33.

The hard layer 34 may also be made of the same material (element) asthat of the hard mask 33, for example, TiN, Ta, or W. When the hardlayer 34 is made of the same material as that of the hard mask 33, theIBE etching rate is mainly determined by the density of the material.More specifically, the IBE etching rate decreases as the densityincreases. Therefore, the hard layer 34 is formed to have a densityhigher than that of the hard mask 33. Consequently, the IBE etching rateof the hard layer 34 can be made higher than that of the hard mask 33.

The film thickness of the hard layer 34 is, for example, 10 (inclusive)to 20 (inclusive) nm. Accordingly, the total film thickness of the hardmask 33 and hard layer 34 is 30 (inclusive) to 50 (inclusive) nm. Aswill be described later, the magnetoresistive element MTJ (the referencelayer 32, tunnel barrier layer 31, and storage layer 30) is processed byIBE using the hard mask 33 and hard layer 34 as masks. This makes thetotal film thickness of the hard mask 33 and hard layer 34 equivalent tothe film thickness of the magnetoresistive element MTJ, and they overlapeach other in a plane.

A distance W between the two adjacent magnetoresistive elements MTJ canbe decreased by decreasing the height H of the hard mask 33. Morespecifically, when the ion beam incident angle (the angle to theperpendicular direction of the film surfaces) is set at 45° in an IBEstep (described later), the distance W between the two adjacentmagnetoresistive elements MTJ can be set to 40 (inclusive) to 50(inclusive) nm if the height H of the hard mask 33 is 40 (inclusive) to50 (inclusive) nm.

An upper electrode 28 is formed on the hard mask 33 and hard layer 34.Since the upper electrode 28 is formed in contact with the hard mask 33made of a metal material, the upper electrode 28 is electricallyconnected to the magnetoresistive element MTJ via the hard mask 33.

Next, an operation example of the magnetoresistive element MTJ will beexplained.

The magnetoresistive element MTJ is, for example, a spin transfer typemagnetoresistive element. Therefore, when writing data or reading datafrom the magnetoresistive element MTJ, electric currents arebidirectionally supplied to the magnetoresistive element MTJ in adirection perpendicular to the film surfaces (stacked surfaces).

More specifically, data write to the magnetoresistive element MTJ isperformed as follows.

When supplying electrons (electrons moving from the reference layer 32to the storage layer 30) from the upper electrode 28, electronsspin-polarized in the same direction as the magnetization direction inthe reference layer 32 are injected into the storage layer 30. In thisstate, the magnetization direction in the storage layer 30 is matchedwith that in the reference layer 32. Consequently, the magnetizationdirections in the reference layer 32 and storage layer 30 are arrangedparallel. In this parallel arrangement, the resistance of themagnetoresistive element MTJ is minimum. This state is defined as, forexample, binary 0.

On the other hand, when supplying electrons (electrons moving from thestorage layer 30 to the reference layer 32) from the lower electrode 27,electrons reflected by the reference layer 32 and spin-polarized in theopposite direction to the magnetization direction in the reference layer32 are injected into the storage layer 30. In this state, themagnetization direction in the storage layer 30 is matched with theopposite direction to the magnetization direction in the reference layer32. Consequently, the magnetization directions in the reference layer 32and storage layer 30 are arranged antiparallel. In this antiparallelarrangement, the resistance of the magnetoresistive element MTJ ismaximum. This state is defined as, for example, binary 1.

Data read is performed as follows.

A read current is supplied to the magnetoresistive element MTJ. Thisread current is set to a magnitude that does not reverse themagnetization direction in the storage layer 30 (to less than the writecurrent). A semiconductor device capable of a memory operation isobtained by detecting the change in resistance of the magnetoresistiveelement MTJ in this state.

Manufacturing Method of First Embodiment

A method of manufacturing the magnetoresistive element MTJ according tothe first embodiment will be explained below with reference to FIGS. 4,5, 6, and 7.

FIGS. 4, 5, 6, and 7 are sectional views showing the manufacturing stepsof the magnetoresistive element MTJ according to the first embodiment.FIGS. 4, 5, 6, and 7 illustrate the steps of manufacturing two adjacentmagnetoresistive elements MTJ.

First, as shown in FIG. 4, a storage layer 30 is formed on an underlayer(not shown) on a lower electrode 27 by, for example, chemical vapordeposition (CVD). The storage layer 30 is a ferromagnetic layer, andmade of a ferromagnetic material containing one or more elementsselected from, for example, Co and Fe. An element such as B, C, or Simay also be added to the ferromagnetic material in order to adjustsaturation magnetization, magnetocrystalline anisotropy, or the like.

Then, a tunnel barrier layer 31 is formed on the storage layer 30 by,for example, CVD. The tunnel barrier layer 31 is a nonmagnetic layer andmade of, for example, MgO.

Subsequently, a reference layer 32 is formed on the tunnel barrier layer31 by, for example, CVD. The reference layer 32 is a ferromagnetic layerand made of, for example, CoFeB. The reference layer 32 may also be madeof a ferromagnetic material containing one or more elements selectedfrom, for example, Co, Fe, B, Ni, Ir, Pt, Mn, and Ru.

A hard mask 33 is formed on the reference layer 32 by, for example, CVD.The hard mask 33 is made of a conductive metal material, for example,TiN. The hard mask 33 may also be a film containing Ti, Ta, or W, or amultilayered film of these materials. The film thickness of the hardmask 33 is, for example, 40 (inclusive) to 50 (inclusive) nm.

After that, the hard mask 33 is patterned by lithography and RIE.Consequently, the planar shape of the hard mask 33 becomes, for example,a circle, and the diameter of the circle is, for example, 20 (inclusive)to 30 (inclusive) nm. Accordingly, the hard mask 33 is formed into apillar shape.

As shown in FIG. 5, a hard layer 34 is formed on the entire surface by,for example, CVD or atomic layer deposition (ALD). More specifically,the hard layer 34 is formed on the upper surface of the reference layer32, and on the upper surface and circumferential surface of the hardmask 33. The hard layer 34 is made of a hard material having an etchingrate of IBE (described later) lower than that of the hard mask 33 (forexample, TiN, Ta, or W). That is, the hard layer 34 is made of a hardmaterial having Vickers hardness higher than that of the hard mask 33.Examples of the hard material are insulating materials such as DLC,microcrystalline Al₂O₃, BN, and SiC. The film thickness of the hardlayer 34 is, for example, 10 (inclusive) to 20 (inclusive) nm.

As shown in FIG. 6, the hard layer 34 is etched back by, for example,RIE. The hard layer 34 is removed from the upper surface of thereference layer 32 and the upper surface of the hard mask 33, andremains on only the circumferential surface of the hard mask 33. Thatis, the hard layer 34 is formed to cover the circumference of the pillarhard mask 33.

In this step, a main gas of RIE is determined accordance with thematerial of the hard layer 34. For example, an O₂-based gas is used asthe main gas when the hard layer 34 is DLC, a BCl₃-based gas is usedwhen the hard layer 34 is Al₂O₃, a Cl₂-based gas or HBr-based gas isused when the hard layer 34 is BN, and an NF₃-based gas is used when thehard layer 34 is SiC.

The film thickness of the hard layer 34 remaining on the circumferentialsurface of the hard mask 33 remains almost unchanged, and is, forexample, 10 (inclusive) to 20 (inclusive) nm. Therefore, the total filmthickness of the hard mask 33 and hard layer 34 is 30 (inclusive) to 50(inclusive) nm.

As shown in FIG. 7, the reference layer 32, tunnel barrier layer 31, andstorage layer 30 are processed by physical etching such as IBE using thehard mask 33 and hard layer 34 as masks. Consequently, the referencelayer 32, tunnel barrier layer 31, and storage layer 30 are patternedlike the hard mask 33 and hard layer 34, i.e., patterned into a circularplanar shape. The diameter of the reference layer 32, tunnel barrierlayer 31, and storage layer 30 is 30 (inclusive) to 50 (inclusive) nm.

In this step, an ion beam of IBE enters in a direction (obliquedirection) inclined to the perpendicular direction of the film surfaces.Accordingly, even when the metal materials contained in the etchedreference layer 32, tunnel barrier layer 31, and storage layer 30 adhereas re-deposition products on the circumferential surfaces of thereference layer 32, tunnel barrier layer 31, and storage layer 30,etching can be performed while removing the re-deposition products. “Anoblique direction” herein mentioned means a direction inclined through θ(0°<θ<90°) to the perpendicular direction of the film surfaces, and θis, for example, 45°.

When the ion beam is incident in the oblique direction as describedabove, the etching of the hard mask 33 by IBE mainly progresses in theoblique direction (from the circumferential surface) as micropatterningadvances.

In this embodiment, the hard layer 34 having a low IBE etching rate isformed on the circumferential surface of the hard mask 33. The hardlayer 34 protects the circumferential surface of the hard mask 33against IBE. Accordingly, while the reference layer 32, tunnel barrierlayer 31, and storage layer 30 are processed by IBE, the hard mask 33and hard layer 34 are not etched from their circumferential surfaces.Also, the etching amount from the upper surface of the hard mask 33 isextremely small. That is, the film thickness of the hard mask 33 afterIBE processing is held almost equal to that before IBE processing. Whenforming the hard mask 33, therefore, its film thickness need only have aminimum necessary value with which the hard mask 33 functions as a mask.

When the ion beam is incident at θ=45° in IBE, the distance W betweenthe two adjacent magnetoresistive elements MTJ can be made as small asthe height H of the hard mask 33. That is, the distance W between thetwo adjacent magnetoresistive elements MTJ can be decreased to 40(inclusive) to 50 (inclusive) nm by setting the height H of the hardmask 33 to 40 (inclusive) to 50 (inclusive) nm.

Then, as shown in FIG. 3, an interlayer dielectric layer 25 made of SiO₂is formed on the entire surface by, for example, CVD. Consequently, theinterlayer dielectric layer 25 is buried between the adjacentmagnetoresistive elements MTJ. After that, the interlayer dielectriclayer 25 formed on the hard mask 33 is planarized and etched back,thereby exposing the upper surface of the hard mask 33. An upperelectrode 28 made of, for example, TiN is formed on the exposed hardmask 33, and electrically connected to the magnetoresistive element MTJ.

Thus, the magnetoresistive element MTJ according to this embodiment isformed.

Effects of First Embodiment

In the above-mentioned first embodiment, when processing themagnetoresistive element MTJ by IBE, the hard layer 34 having an IBEetching rate lower than that of the hard mask 33 made of a metalmaterial is formed on the circumferential surface of the hard mask 33.Since the hard layer 34 protects the hard mask 33, the etching amount ofthe hard mask 33 by IBE can be reduced. That is, the film thickness H ofthe hard mask 33 can be minimized within the range over which the hardmask 33 can function as a mask. As a consequence, it is possible toreduce the dimensional variations of the hard mask 33 and hence thedimensional variations of the magnetoresistive element MTJ.

In addition, it is possible to minimize an obstacle to the ion beamincident in the oblique direction in IBE by minimizing the filmthickness H of the hard mask 33. Accordingly, adjacent magnetoresistiveelements MTJ can be processed by IBE even when the distance W betweenthem is decreased. That is, the density of the magnetoresistive elementsMTJ can be increased.

Second Embodiment

A magnetoresistive element MTJ according to the second embodiment willbe explained below with reference to FIGS. 8, 9, 10, and 11. In thefirst embodiment, after the hard mask 33 made of a metal material ispatterned, the hard layer 34 is formed on the circumferential surface ofthe hard mask 33, and the hard mask 33 and hard layer 34 are used asmasks in IBE processing. In the second embodiment, however, a hole 42 isformed in a sacrificial layer 40, and a hard layer 34 and hard mask 33are formed in this order on the inner surface (circumferential surface)of the hole 42. That is, the second embodiment is an example in whichthe hard mask 33 and hard layer 34 are formed by a so-called damascenemethod. The magnetoresistive element MTJ according to the secondembodiment will be explained in detail below.

Note that in the second embodiment, the same features as those of theabove-mentioned first embodiment will be omitted, and different featureswill be explained.

Manufacturing Method of Second Embodiment

A method of manufacturing the magnetoresistive element MTJ according tothe second embodiment will be explained below with reference to FIGS. 8,9, 10, and 11.

FIGS. 8, 9, 10, and 11 are sectional views showing the manufacturingsteps of the magnetoresistive element MTJ according to the secondembodiment. FIGS. 8, 9, 10, and 11 illustrate the steps of manufacturingtwo adjacent magnetoresistive elements MTJ.

First, as shown in FIG. 8, a storage layer 30 as a ferromagnetic layeris formed on an underlayer (not shown) on a lower electrode 27 by, forexample, CVD. A tunnel barrier layer 31 as a nonmagnetic layer is formedon the storage layer 30 by, for example, CVD. A reference layer 32 as aferromagnetic layer is formed on the tunnel barrier layer 31 by, forexample, CVD.

Then, a sacrificial layer 40 is formed on the reference layer 32 by, forexample, CVD. The sacrificial layer 40 is made of, for example, SiO₂,but is not limited to this material. The sacrificial layer 40 need onlybe made of a material which can be processed by IBE or the like moreeasily than a hard mask 33, and by which etching selectivity between ahard mask 33 and hard layer 34 is obtained in a step (described later)of removing the sacrificial layer 40. The film thickness of thesacrificial layer 40 is equivalent to that of a hard mask 33 to beformed later, for example, 40 (inclusive) to 50 (inclusive) nm.

Subsequently, as shown in FIG. 9, a resist 41 is formed on thesacrificial layer 40. After that, the resist 41 is patterned bylithography. A hole 42 is formed in the sacrificial layer 40 by RIEusing the patterned resist 41 as a mask. The hole 42 is formed to extendthrough the sacrificial layer 40, and the reference layer 32 is exposedto the bottom surface of the hole 42. The hole 42 has, for example, acircular planar shape, and has a diameter of, for example, 30(inclusive) to 50 (inclusive) nm. The diameter of the hole 42 is thetotal diameter of a hard mask 33 and hard layer 34 to be formed later,and the diameter of the magnetoresistive element MTJ.

As shown in FIG. 10, the resist 41 is removed, and a hard layer 34 isformed on the entire surface by, for example, CVD or ALD. Morespecifically, the hard layer 34 is formed on the inner surface of thehole 42 (on [the upper surface of] the reference layer 32 and on [thecircumferential surface of] the sacrificial layer 40 in the hole 42),and on (the upper surface of) the sacrificial surface 40 outside thehole 42. The film thickness of the hard layer 34 is, for example, 10(inclusive) to 20 (inclusive) nm.

After that, the hard layer 34 is etched back by, for example, RIE.Consequently, the hard layer 34 is removed from the upper surface of thereference layer 32 and the upper surface of the sacrificial layer 40,and remains on only the circumferential surface of the sacrificial layer40.

Then, as shown in FIG. 11, a hard mask 33 is formed on the entiresurface by, for example, CVD. More specifically, the hard mask 33 isformed on (the circumferential surface of) the hard layer 34 in the hole42 so as to fill the hole 42, and on (the upper surface of) thesacrificial layer 40 outside the hole 42. After that, the hard mask 33on the upper surface of the sacrificial layer 40 outside the hole 42 isplanarized and removed by, for example, chemical mechanical polishing(CMP). Thus, the hard mask 33 is buried in only the hole 42. That is,the pillar hard mask 33 and the hard layer 34 covering the circumferenceof the hard mask 33 are formed in the hole 42.

After the sacrificial layer 40 is removed by, for example, RIE, as shownin FIG. 7, the reference layer 32, tunnel barrier layer 31, and storagelayer 30 are processed by physical etching such as IBE using the hardmask 33 and hard layer 34 as masks.

Note that the sacrificial layer 40 may also be removed by IBE forprocessing the reference layer 32, tunnel barrier layer 31, and storagelayer 30.

Steps after that will be omitted because they are the same as those inthe first embodiment. In this manner, the magnetoresistive element MTJaccording to the second embodiment is formed.

Effects of Second Embodiment

The above-mentioned second embodiment can achieve the same effects asthose of the first embodiment.

In addition, in the second embodiment, the hole 42 is formed in thesacrificial layer 40 made of, for example, SiO₂, the hard layer 34 isformed on the inner surface of the hole 42, and the hard mask 33 isburied. That is, the pillar hard mask 33 and the hard layer 34 aroundthe hard mask 33 are formed without patterning the hard mask 33. Sincethe sacrificial layer 40 can be processed by RIE more easily than thehard mask 33, the hole 42 can be formed more easily than patterning ofthe hard mask 33. That is, the second embodiment can make themanufacturing process easier than that of the first embodiment.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetoresistive element manufacturing methodcomprising: sequentially forming a first ferromagnetic layer, a tunnelbarrier layer, and a second ferromagnetic layer on a substrate; forminga sacrificial layer on the second ferromagnetic layer; forming a holeextending through the sacrificial layer; forming a hard layer on thesacrificial layer in the hole; forming a conductive hard mask on thehard layer in the hole so as to fill the hole; removing the sacrificiallayer; and processing the second ferromagnetic layer, the tunnel barrierlayer, and the first ferromagnetic layer by IBE in an oblique directionby using the hard mask and the hard layer as masks, wherein the hardlayer has an IBE etching rate lower than that of the hard mask.
 2. Themethod of claim 1, wherein a Vickers hardness of the hard layer ishigher than that of the hard mask.
 3. The method of claim 1, wherein thehard layer comprises one of DLC, Al₂O₃, BN, and SiC.
 4. The method ofclaim 1, wherein the hard mask comprises one of TiN, Ta, and W.
 5. Themethod of claim 1, wherein the hard layer and the hard mask comprisesthe same material, and a density of the hard layer is higher than thatof the hard mask.
 6. The method of claim 5, wherein the hard layer andthe hard mask comprise one of TiN, Ta, and W.
 7. The method of claim 1,wherein the forming the hard layer on the sacrificial layer in the holecomprises: forming the hard layer on an entire surface; and removing thehard layer from an upper surface of the second ferromagnetic layer andan upper surface of the sacrificial layer by RIE.